Titration with 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) as Evidence for Several Photophosphorylation Sites in Scenedesmus

Abstract Photophosphorylation was measured in intact cells of Scenedesmus obtusiusculus, which were made phosphate starved before the start of the experiments. Photophosphorylation was titrated with narrow intervals of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) from 10‐7M upwards. Plots of the reciprocal of photophosphorylation against concentration of DCMU gives three essentially straight lines; one between 10‐7 and 10‐6M DCMU; one between 10‐6 and 6 · 10‐6M DCMU; and one for more than 6 · 10‐6M DCMU, the last‐mentioned line being parallel to the abscissa. The stoichiometry between the three reactions is roughly 2: 1: 1. At least three sites for photophosphorylation are indicated, and the assumption that all sites work with approximately the same efficiency would make them four.